`fail!()` in proc gives internal error

Reported by mtikekar@

https://groups.google.com/g/xls-dev/c/LgPOsMdDG2w/m/vi2ty7idBgAJ?utm_medium=email&utm_source=footer

```dslx
fn out_fn(x: u32, state: u32) -> u32 {
match x {
u32:0 => fail!("bad_x", state),
_ => x + state
}
}
proc Fmac {
input_a_consumer: chan in;
output_producer: chan out;

init { u32:0 }

config(input_a_consumer: chan in, output_producer: chan out) {
(input_a_consumer, output_producer)
}

next(tok: token, state: u32) {
let (tok, input_a) = recv(tok, input_a_consumer);
let result = out_fn(input_a, state);
let tok = send(tok, output_producer, result);
result
}
}
```